Look into the attachment, if you're in the mood. Otherwise: it's only half the way. In fact, I think, one can change this with similar effort to have FTP users without e-mail as well.
So instead of a checkbox - like in the attached solution - one could use an option field on the user tab, represented in the db as an char(1) field:
- => E-mail only
- => FTP & E-mail
- => FTP only
Then: only if the option value is > 1 you would define the /dev/false user directory in the password file. If the value is > 2 you would NOT write into virtualusertable of postfix.
Backward compabilty: for all already existing users the option '2' could be set. Make that sense?